1. Field of the Invention
This invention relates to processes for replacing anions of organic cationic salts, and more particularly to processes for replacing anions of cationic salts, particularly dye salts, with solubilizing anions.
2. Description Relative to the Prior Art
It is well known in the art to employ organic cationic salts for many purposes. Organic cationic salts include dyes, such as spectral sensitizing dyes and photobleachable dyes; development accelerators and development restrainers; ionic surface active agents; mordants; photographic stabilizers; bleachable antihalation dyes; optical filters and the like. Many of the physical and chemical characteristics of organic cationic salts, such as the optical properties of cationic dye salts, are determined by the organic cationic portion of the molecule and are essentially independent of the anion. However, the anion plays an important role in determining the solubility of the salt. For example, organic cationic salts are frequently isolated as perchlorate salts or tetrafluoroborate salts, which exhibit low solubility and good crystallinity. However, the solubility of these salts is too low for many applications, such as in coating very thin layers having a high organic cationic salt concentration. In order to increase the solubility, anions such as perchlorate or tetrafluoroborate are replaced with anions which increase the solubility of the cationic salts in organic solvents.
Several methods of replacing anions of organic cationic salts are known. One method comprises treating a water-soluble organic cationic salt, such as a pyrilium salt, with the acid or salt of the desired anion. However, this method is not applicable to all organic cationic salts, such as these which are water-insoluble.
Another method comprises treating an iodide salt of an organic cation with propylene oxide and the acid of the desired anion, as reported in Research Disclosure 14831 (1976). However, this method is applicable only to the iodide and some bromide salts of pyrylium and cyanine dyes.
The use of an ion exchange resin is described by Research Disclosure 14831 and by a publication of Curran and Strohl, Phot. Sci. Eng., 21, 148 (1977). However, this method requires large quantities of solvent to dissolve organic cationic salts having limited solubilities, and is extremely time consuming. Further, impurities in the final product can be introduced by the exchange resin, and the method is characterized by low yields.
In another procedure, an adduct of the organic cationic salt is isolated and then reacted with an acid of the desired anion. This procedure is only applicable if the organic salt forms a suitable adduct and, even then, the basic conditions employed may cause side reactions which contaminate the product.
It is thus seen that a process for conveniently replacing commonly encountered anions of both water-soluble and water-insoluble organic cationic salts with solubilizing anions is extremely desirable. It is further seen that such a process should produce an organic cationic salt of high purity and in high yields.